1. Field of the Invention
This invention relates to numerical control devices to ensure driving of the members of a single spindle automatic lathe, the operations of which are mechanically produced by a camshaft.
2. Description of the Prior Art
Before the rather recent coming out of the numerical control devices, the lathes of the above mentioned type were the only ones at disposal to machine in a fully automatic manner series of identical workpieces out of bars of stock.
As soon as the numerical control devices cam out, the lathe producers considered the creation of a new type of automatic lathes, especially designed in function of the new control mode. The user of such a lathe thus can undertake small series of workpieces, even prototypes at costs similar to those of great series, because setting such a machine does not take much time. For the workman familiar with automatic lathes, the diagram of the required operations to machine a given workpiece is soon established and its typing in the computer memory of the numerical control device is immediate.
The cost of these automatic lathes is, however, high so that there are generally much less than 20% of the same to be found in the equipment of the lathe-turning factories. For many reasons it would anyway not be judicious if such a factory changed its whole equipment to numerically controlled lathes.
Machining great series of workpieces having complicated shapes and therefore requiring the use of more than ten tools does not pay on a numerically controlled lathe. Moreover, a mechanical control by means of cams is more reliable than an electronic control; it has less troubles. In addition thereto, if with a mechanically controlled lathe a breakdown nevertheless occurs, the user can himself execute the repair. On the contrary, a failure of an electronic control necessitates the service of a foreign expert, if not even of the machine producer himself, thus at least causing a relative long stop of the production, while the user is compelled to a complete passivity.
The use of numerically controlled lathes has, however, pointed out that the conventional lathes controlled by cams had shortcomings of which the users were more or less aware, however without measuring the real extent thereof. Anyway, the users of numerical control devices immediately eliminated these shortcomings by programming their lathes so as to ensure the most favorable cutting speed of every tool.
Automatic lathes being mechanically controlled by cams and in which the spindle can be driven at two different speeds are, however, known in the art, but they are strictly limited to that possibility.
There are also known in the art automatic lathes, in particular multi-spindle automatic lathes, in which the camshaft is driven by an independent motor subjected to an electronic device which is sensitive to the angular positions of the camshaft and which accelerates the rotational speed of that motor and consequently of the camshaft when the tools have to be moved either from their rest position into that in which they are ready to undertake their machining operation, or from that in which they have just performed their operation into their rest position (U.S. Pat. No. 4,253,359). The electronic device of these known lathes thus simply switches over the rotational speed of the camshaft motor from a high speed, for the approach movement of the tools, to a lower speed, for the operation thereof, and then again from that low speed to the high speed, for the return movement of the tools. That known device does, however, in no way affect the motor driving the spindles which consequently always rotate at the same speed. Now, it is precisely the spindle speed which should be varied, and that, not to abruptly change over from a predetermined speed to another predetermined one, but to progressively change over from one speed to another one in a continuous manner.
There are also known in the art machines, such as presses or machines comprising transfer mechanisms, in which the conventional control devices with cams or with switching boxes are replaced by an electronic control device with almost instantaneous reactions in order to increase the working rythm as a whole (U.S. Pat. No. 4,120,185). These known control devices thus do not control the machines with which they are associated so that they would work at different speeds during a cycle of operations, but so that they work at a unique high speed.
According to the shape of the pieces which have to be machined on an automatic lathe, the tools which would be more suitable to that end, would, however, require more than two different speeds of rotation of the lathe spindle, because of their cutting speeds. Moreover, to carry out a radially directed recessing feed of a tool, especially to sever the finished workpiece from the bar of stock, the rotational speed of the lathe spindle should progressively be accelerated as the tool comes nearer to the workpiece axis, in order that the cutting speed remains constant. Since the known automatic lathes being mechanically controlled by cams do not permit these variations of the lathe spindle speed during a turning operation, their users hitherto had to resort to less favorable tools to carry out some machining operations, viz. to tools which soon lose their edge to the prejudice of the production.